add condstore & qresync imap extensions

for conditional storing and quick resynchronisation (not sure if mail clients are actually using it that).

each message now has a "modseq". it is increased for each change. with
condstore, imap clients can request changes since a certain modseq. that
already allows quickly finding changes since a previous connection. condstore
also allows storing (e.g. setting new message flags) only when the modseq of a
message hasn't changed.

qresync should make it fast for clients to get a full list of changed messages
for a mailbox, including removals.

we now also keep basic metadata of messages that have been removed (expunged).
just enough (uid, modseq) to tell client that the messages have been removed.
this does mean we have to be careful when querying messages from the database.
we must now often filter the expunged messages out.

we also keep "createseq", the modseq when a message was created. this will be
useful for the jmap implementation.

test-upgrade.sh

@@ -9,9 +9,11 @@
 set -e
 # set -x
 
-# We'll allow max 256mb of memory during upgrades. We modify the softlimit when
+# We'll set a max memory limit during upgrades. We modify the softlimit when
 # importing the potentially large mbox file.
-ulimit -S -d 256000
+# Currently at 768MB, needed for upgrading with 500k messages from v0.0.5 to
+# v0.0.6 (two new indexes on store.Message).
+ulimit -S -d 768000
 
 (rm -r testdata/upgrade 2>/dev/null || exit 0)
 mkdir testdata/upgrade
@@ -59,7 +61,7 @@ for tag in $tags; do
 			echo 'Importing bulk data for upgrading.'
 			gunzip < ../upgradetest.mbox.gz | time ./$tag/mox ximport mbox ./stepdata/accounts/test0 upgradetest /dev/stdin
 			echo
-			ulimit -S -d 256000
+			ulimit -S -d 768000
 		fi
 
 		echo "Upgrade data to $tag."